This invention relates to a method and apparatus for the separation of air.
The separation of air by rectification is very well known indeed. Rectification is a method in which mass exchange is effected between a descending stream of liquid and an ascending stream of vapor such that the ascending stream of vapor is enriched in a more volatile component (nitrogen) of the mixture to be separated and the descending stream of liquid is enriched in a less volatile component (oxygen) of the mixture to be separated.
It is known to separate air in a double rectification column comprising a higher pressure rectification column which receives a stream of purified, compressed, vaporous air at a temperature suitable for its separation by rectification, and a lower pressure rectification column which receives a stream of oxygen-enriched liquid air for separation from the higher pressure rectification column, and which is in heat exchange relationship with the higher pressure rectification column through a condenser-reboiler, of which the condenser provides liquid nitrogen reflux for the separation and the reboiler provides an upward flow of nitrogen vapor in the lower pressure rectification column.
There is a net requirement for refrigeration to be provided to the air separation. At least part of this requirement arises from the operation of the double rectification column at cryogenic temperatures. At least part of this requirement for refrigeration is conventionally met by expanding with the performance of external work a part of the incoming air flow or a part of a nitrogen product of the separation.
It is known that the thermodynamic efficiency with which the double rectification column operates can be enhanced by condensing a part of the flow of air to be separated and introducing a stream of resulting liquid air into the higher pressure rectification column at an intermediate mass exchange level thereof.
The improvement in efficiency results from a reduction that can be made in the liquid nitrogen reflux supplied to the top of the higher pressure rectification column. It is similarly advantageous to introduce a stream of liquid air into the lower pressure rectification column at an intermediate mass exchange level thereof.
The condensation of the air does of course introduce a further source of thermodynamic inefficiency into the air separation method. It is therefore desirable to integrate the condensation of the air into the method in such a way that the increased thermodynamic efficiency with which the double rectification column operates outweighs the additional thermodynamic inefficiency introduced by the condensation of the air.